1. Field of the invention
The present invention relates to an air shutter to be installed at the entrance where temperature difference exists between inside and outside thereof for preventing air flow through the entrance, that is, the intrusion of outside air and extrusion of inside cold air.
2. Description of the Related Art
An air curtain producing apparatus for preventing the intrusion of outside air through the doorway of the thermal insulation door used in a building is disclosed in Japanese Patent No. 37-12183.
Said invention proposes an air curtain producing apparatus composed such that air is spouted out from both sides of the doorway in a direction horizontal or slanted downward to form two air streams flowing in the direction opposite to each other, each air stream being located parallel to each other in the direction perpendicular to the entrance plane, a whirl stream being generated between said two air streams.
However, with said apparatus, a problem is involved that the amount of air to form the air streams is larger compared with an air curtain producing apparatus in which an air curtain is formed with the air spouted out from one side of the doorway.
Among proposals disclosed afterward, there is a proposal disclosed in Japanese Patent Laid-Open publication No. 51-118149. The proposed air curtain producing apparatus is composed such that, as seen in FIG. 9, the circulation air transferred through a fan 110 provided in the top part of a portal-shaped duct 106 of for forming an air curtain is changed in its flow direction by 180 degrees in the vertical part of the duct by guides 108, so the air is spouted out in the direction slanting downward particularly in the region near the top of the portal-shaped duct where the velocity of the spouting air is large. Therefore, there is a problem that air curtain is difficult to be formed in the upper region of the entrance and the effect of preventing the intrusion of outside warm air in the upper region of the entrance is weak.
Further, with the apparatus according to the proposal, since the recirculation fan 110 is located in the upper part of the portal-shaped duct 106, the maintenance of the thermal insulating door not shown in the drawing is difficult.
Another example of air curtain producing apparatus proposed in the past is shown in FIG. 10A and FIG. 10B showing a section taken along lines □-□ in FIG. 10A. According to the proposal, a portal-shaped duct 53, in which an air recirculation passage 55 is formed and an air recirculation fan 54 is located in the passage, is provided outside thermal insulation doors 57, 57. The doors can be slid to be opened or closed while contacting a seal member of rubber or vinyl provided between the rear faces of the doors and the outer face 51 of the wall 50 of a building. An air curtain 56 is formed between the opposing vertical side faces 53a, 53a of the duct 53 by spouting out air from a slit provided in the face 53a of the left side vertical duct part of the duct 53 and sucking the air from a slit provided in the face 53 of the right side of the vertical duct part of the duct 53 to prevent the intrusion of outside air into the building.
Each of clearances 53b, 53b between the rear side face of each of the vertical duct parts of the duct 53 and each of said thermal insulation doors is sealed by means of a seal member of rubber or vinyl.
With the apparatus described above, when the thermal insulation doors 57, 57 are slid toward left and right to open the entrance as shown in FIG. 10B, even if the air curtain 56 is formed in front of the space 65a surrounded by a broken line formed between said doors when they are opened, the intrusion of outside air through the clearance formed above the top of the space 65a can not be prevented. Therefore, sufficient effect of interception of outside air cannot be achieved.
Still another proposal is disclosed in Japanese Patent Laid-Open publication No. 5-73436. According to the proposal, as shown IN FIG. 11A and FIG. 11B showing a section taken along lines VII-VII in FIG. 11A, thermal insulating doors 60, 61 are provided for slide outside the wall 50 of a building, the doors being packed with insulation material 52, a seal member 51 being provided around the opening of the wall 50. Sirocco fans 60a and 60b are provided to the left and right end of the thermal insulating doors 60a and 60b respectively. Outside air sucked by the fan 61a passes through the air passage formed outside the right door 61 is sprouted out therefrom toward the air passage formed outside the left door 60 and sucked thereinto by the fan 60a in order to form an air curtain 62 in front of the space 65b between both the doorway side ends of the opened doors 60, 61. The air is exhausted from the fan 60a toward outside.
With the proposal, although an air curtain 62 is produced in front of the space 65b to intersect air flow between the outside and inside of the building, there remains a problem that the intrusion of outside air from the clearance above the top of the space 65b cannot be prevented.
Further, with the proposal, the temperature of the interception air stream is considerably higher compared with that of the inside cold air because the air stream consists of the outside air sucked from the right side of the thermal insulation door 61, and white smoke tends to be generated due to the condensation of the moisture in the air stream as a result of the contact or mixing of the air stream with the inside cold air. The white smoke induces poor visibility from outside. There occurs also a problem that ice is formed on the floor and on the doorway-side end faces of the door 60 and 61 where the inside cold air mixes with the air stream which consists of only outside air of higher temperature.
As has been mentioned above, with lateral flow type air curtain producing apparatuses of prior art, there is a problem that outside air intrudes from the clearance formed above the top of the opened space of doorway when doors are opened.
In FIG. 10C is shown in a sectional view a down-flow type air curtain producing apparatus which has been used widely hitherto.
As can be seen in the drawing, a down-flow type air curtain producing apparatus 63 is of a simple construction, which is provided above the outer wall 50 of a building and produces interception air stream 63a flowing downward over the top of the door 64, has been widely used because of its simplicity. However, the interception effect is decreased near the floor where the velocity of the air stream is lowest, and white smoke is generated due to the condensation of the moisture in the air stream as a result of mixing of it with the cold air leaking out from inside, inducing poor visibility from outside and also ice is formed on the floor.
With the down-flow type air curtain, as the flow angle of air is constant, stable curtain effect is difficult to be obtained, and when the difference in air temperature between the inside and outside of the building, the effect of interception near the bottom part of the air curtain decreases. Particularly in the bottom region of the air curtain where the air velocity is small, the air curtain tends to be broken.
As the down-flow type air curtain involves problems as mentioned above, it is difficult to sufficiently prevent the cold air from leaking out from inside.
To solve the problems mentioned above, the inventers of the present application proposed in Japanese Patent Laid-Open Publication No. 2000-249382 an air curtain producing apparatus, with which the problem of the decrease of interception effect near the end of the down-flowing air stream as is observed in the down-flow type air curtain of prior art and the problem of the condensation due to the leakage of cold air observed in the lateral flow-type air curtain are solved, and an air curtain with high efficiency in interception and without the occurrence of condensation of moisture can be produced.
According to the proposal, as shown FIG. 12, the apparatus is composed as follows:    (a) Each of the thermal insulation door leaf 71 and 72 of a double-leaf door is provided in the doorway side thereof an air spouting opening 76 and 73, an air sucking opening 74 and 77, and an air circulation fan 83 and 84 respectively in order to produce a circulating interception air stream comprising a lower side interception air stream 78a and an upper side interception air stream 78b is formed.    (b) That is, a cold air interception air stream 78a is formed in the lower region of the doorway to prevent the cold air in the inside from leaking out, a hot air interception air stream 78b is formed in the upper region of the doorway to prevent the hot air (outside air) from intruding into the inside, and the air of said interception air streams circulates from the one to the other stream.    (c) Further, a down-flow air stream 79 is formed using a part of the circulating air flowing in the upper part of the doorway.    (d) The air spouting out angle of the lower interception air stream, i.e. the cold air interception air stream 78a is inclined toward inside and the angle of inclination is variable depending on conditions in order to achieve higher interception efficiency.
As described above, according to the proposal, a down-flow air stream 79 and a lateral-flow hot air interception air stream 78b are formed in order to prevent outside air from intruding inside, and a lateral-flow cold air interception air stream 78a is formed in order to prevent the cold air in the inside 80 from flowing out to the outside, the air of the down-flow air stream and the hot air interception air stream 78b being introduced to the fan 83 to form the cold air interception air stream 78a. 
According to the proposal, although it is possible to prevent the occurrence of white smoke due to the condensation of the moisture in outside hot air as occurred with apparatuses of prior art by forming the circulating interception air stream comprising the hot air interception air stream and cold air interception air stream, the problem concerning efficient interception between inside and outside was not solved enough.